The numerical simulator "SHIFT", which simulates performances of fractured basement reservoirs during hydraulic stimulations, has been developed. Hydraulic injections are commonly conducted in geothermal reservoirs for a purpose of reservoir stimulation. This treatment is studied vigorously in Hot Dry Rock (HDR) geothermal programs and many success stories are reported. This treatment uses a large volume of slick water with no proppant. The mechanism of permeability enhancement is recognized as shear dilation, which is associated with shear sliding induced by elevated fluid pressure. Once sliding occurs along a pre-existing fracture, asperities of the fracture keep the opening along the plane (self-propping) to maintain permeability. SHIFT simulates the shearing of fractures and the relating permeability change in a dynamic process by coupling the fluid flow analysis and the shear dilation analysis. The simulation is based on a discrete fracture network model, in which pressure distribution and permeability enhancements are calculated simultaneously. The output includes a history of injection pressure, microseismic (AE) activity relating to the shearing along pre-existing fractures.
The paper describes the concept of the SHIFT and its performance by showing results of the case study for the fractured basement reservoir in Hokkaido, Japan.

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